"Once he had been handcuffed he was sat up. They were looking to transport him back to the police station but became concerned at his condition."
Mr van der Velde couldn't say what was wrong with the man or in what way he had earlier been restrained.
The two police officers called an ambulance and asked Ms Marston, a nurse, to check the man.
Ambulance staff tried to revive the man on arrival but he was pronounced dead at the scene.
The 23-year-old was taken to Waikato Hospital and Mr van der Velde said the younger man was in shock.
He said police expressed their condolences to the Hamilton-based family of the dead man.
"The deceased's family are working with the police and dealing with the death as most families would where they've lost a son."
It was not clear if the man had any pre-existing medical conditions.
Mr van der Velde said the two police officers were assisting the investigation and would be offered counselling.
"Any situation like this is traumatising. Where you encounter someone who is alive and they die in your presence is traumatising.
Mr van der Velde said the occupants of the house were co-operating with the inquiry, which was now headed by an independent senior investigator from outside the district.
The Independent Police Conduct Authority had also been advised and the death had been reported to the coroner.
The body of the dead man, who was known to police, was removed from the house yesterday and an autopsy was likely to be held today.
Miss Marston did not want to comment when contacted by the Herald. The 21-year-old received more than $40,000 in donations and a car when she went public with Marley's plight and her financial hardship this year.